Engineers and other scientists are constantly seeking new strategies to reduce undesirable emissions from internal combustion engines. Among these undesirable emissions are particulate matter, NOx and unburned hydrocarbons. One strategy that has proven promising for drastically reducing these undesirable emissions, especially NOx, is known as homogeneous charge compression ignition (HCCI). This strategy typically involves mixing distillate diesel fuel with air in an engine cylinder before autoignition conditions arise, with the aim of causing the mixture to combust at about top dead center. Due in part to combustion occurring at relatively lower temperatures without a flame front or locally rich concentrations of fuel, this strategy can produce extremely low emissions. However, an HCCI strategy creates new problems that must be overcome if the engine is to have the ability to compete performance wise with typical diesel engines.
One problem that has been particularly difficult in overcoming relates to the ability to operate an HCCI engine at relatively high loads. Due in part to the relatively high reactivity of commercially available distillate diesel fuel having a cetane number on the order 45 to 55 and its associated relatively short ignition delay, and the inherent limits associated with controlling ignition timing, premature ignition at high loads before top dead center can sometimes occur. When this happens, extremely high pressure rise rates can occur in the engine cylinder. These extreme pressure rise rates can often exceed the structural integrity limits of the engine, possibly to the extent of destroying head gaskets and even breaking the head free of the engine block in extreme circumstances.
Thus, limitations for achieving HCCI at high loads can be attributed in part to premature ignition of the charge prior to top dead center, such that combustion is accompanied by compression. As stated earlier, this leads to substantial pressure rise rates and sub-optimal combustion phasing from a thermo dynamic standpoint. Various strategies, such as cooled exhaust gas recirculation, reduced compression ratio, and injection timing can be effective in suppressing ignition, and hence, prevent excessive rise rates at higher loads. However, these strategies each compromise thermal efficiency. Furthermore, low compression ratios can make cold start of an engine a serious challenge. The use of heavy amounts of exhaust gas recirculation (EGR) also poses a significant challenge to current air system technologies.
The present disclosure is directed to overcoming one or more of the problems set forth above.